Method for temporal synchronisation of at least two measuring computers cooperating over a telecommunication network such as internet, intranet or similar

ABSTRACT

A method for time synchronization of a number of measuring computers cooperating over a telecommunications network includes providing a number of time sources associated with one of the measuring computers. Each of the time sources has a different accuracy and can provide a time stamp. Using the first measuring computer, one of the time sources is selected as a function of the accuracy of the time source.

The present invention relates to a method for time synchronization in atleast two measuring computers cooperating over a telecommunicationsnetwork such as Internet, intranet or similar, using selection of a timesource, and to a device for carrying out the method.

BACKGROUND

A measuring system for measuring the Internet Protocol (IP) performanceparameters, such as one-way delay, IP delay variations, and packetlosses, in IP networks is known from non-prepublished German PatentApplication DE 100 46 240.5. The subject matter of non-prepublishedGerman Patent Application DE 101 28 927.8 is a method that allows timestamps to be generated in the underlying measuring system even whenaccess to a reference clock is blocked for a short time.

The measuring system underlying these patent applications is adistributed measuring system, i.e., the individual system components arespatially distributed and interconnected via a telecommunicationsnetwork. This measuring system includes at least two measuringcomputers, a database in which the measurement results and theconfiguration of the measuring system are stored, a control computercontrolling the measuring computers for determining the measurementresult, as well as various graphical user interfaces, in particular forconfiguring the measuring system and visualizing the obtainedmeasurement results.

In order to carry out the measuring method, a unidirectional measurementpath is established between at least two measuring computers. On thismeasurement path, measurement packets are sent from a first measuringcomputer to a second measuring computer with a configurable distributionin time.

In the process, the departure of the measurement packet from the firstmeasuring computer is recorded; i.e., a first time stamp is generated.This first time stamp is transmitted to the second measuring computertogether with the measurement packet and other data, such as sequencenumbers. The second measuring computer records the arrival of themeasurement packet and generates a second time stamp. To allow theone-way delay resulting from the difference of the two time stamps to bedetermined with sufficient accuracy, the time stamps generated by themeasuring computers need to be time-synchronized with sufficientaccuracy.

A technical implementation is, for example, the generation of the timestamps using a satellite system, such as GPS (Global PositioningSystem), acting as a time source. In the process, the measuringcomputers continuously receive, via a GPS antenna, the UTC time(Universal Coordinated Time) transmitted by a plurality of satellites.Using a GPS map integrated into the measuring computers, it is thuspossible to generate time stamps with an error of +/−0.5 μs.

The GPS satellite system used as a timer, and the further components GPSantenna and GPS map are together more simply referred to as GPS clockhereinafter.

The measurement results are retrieved by the control computer from thesecond measuring computer as measured data and stored in a database,where they are made available for visualization. The measurement resultsand the system status may optionally be displayed via an offline displayor an online display. In this context, “offline display” means that thedisplay of the measurement results must be initiated manually via a WWWbrowser while in the case of the online display, the display isautomatically updated and displayed at a certain time interval.

The above-mentioned graphical user interfaces are used for this purpose.

The configuration of the measuring system is also carried out using theaforementioned graphical user interface. To this end, the user entersinformation about the type and course of the measurement. Theinformation entered is stored in a database; the control computer readsthis data from the database, configures the measuring computersaccordingly, and starts or stops the measurement connections accordingto this data.

As mentioned earlier, it is of outstanding importance for the quality ofthe obtained measurement result that the first and second time stamps betime-synchronized with sufficient accuracy. Should the first and secondtime stamps not be synchronized with sufficient accuracy, the measuredone-way delay as the difference of the two time stamps can consequentlynot be exactly determined either.

SUMMARY OF THE INVENTION

In this context, it turns out to be particularly disadvantageous thatwhen the GPS clock fails, for example, due to problems with the GPSantenna, contact problems in the antenna feeder, or the like, nomeasurement can be performed because of the lack of the time stamp.

It is an object of the present invention to provide a method for timesynchronization of at least two measuring computers cooperating over atelecommunications network such as Internet, intranet or similar, insuch a manner that a measurement can be performed even when the GPSclock fails, while avoiding the above-mentioned disadvantages.

The present invention provides a method for time synchronization of aplurality of measuring computers cooperating over a telecommunicationsnetwork. The method includes:

-   -   providing a plurality of first time sources associated with a        first measuring computer, each of the first time sources having        a different respective accuracy and configured to provide a        first time stamp; and    -   selecting, using the first measuring computer, a third time        source of the plurality of first time sources as a function of        an accuracy of the third time source.

The present invention also provides a time synchronization device. Thetime synchronization device includes: a first measuring computer; asecond measuring computer cooperating with the first measuring computerover a telecommunications network; and a plurality of first time sourcesassociated with a first measuring computer, each of the first timesources having a different respective accuracy and configured to providea first time stamp. The first computer is configured to select a thirdtime source of the plurality of first time sources as a function of anaccuracy of the third time source.

The present invention includes the discovery that by providing aplurality of independent time sources at the individual measuringcomputers, the probability that no time source can be read is minimized,thus ensuring that a time stamp is read out.

Therefore, in accordance with the present invention, several timesources of different accuracy are made available to each measuringcomputer for reading the time stamp from a time source. The selection ofthe time source to be used for generating the required time stamp ismade by the measuring computer as a function of the accuracy of theavailable time sources. This redundancy of time sources has theadvantage that the generation or the readout of a time stamp from a timesource is ensured in a simple manner. The risk of a measurement failuredue to the lack of a time stamp is minimized by ensuring that the timestamp is read from a second time source in the case that a first timesource fails.

To obtain the best possible measurement results, the measuring computerfirst selects the time source of the highest accuracy for reading thetime stamp from a time source.

If the measuring computer is unable to read a time source of higheraccuracy, it automatically selects a time source of the next bestaccuracy. This hierarchical method with regard to the selection of thetime source allows the best possible measurement result to be obtainedunder the given circumstances, i.e., the failure of a more accurate timesource.

In accordance with one embodiment of the present invention, signals of asatellite system, such as GPS (Global Positioning System), are used asthe time source of the highest accuracy.

The signals of the satellite system are received by local GPS receiversintegrated into the measuring computers. The GPS receiver, whichincludes, inter alia, a GPS map and a GPS antenna as components, will bemore simply referred to as “GPS clock” hereinafter. Using a GPS clock asthe time source of the highest accuracy, a tolerance of +/−0.5 μs isensured for the readout of the time stamp in a simple manner.

Preferably, the measuring computers each have local clocks that arecontinuously synchronized to the local GPS receivers via NTP (NetworkTime Protocol)—internal synchronization. Internal synchronization viaNTP provides a simple way to generate a second, highly accurate timesource.

These internally synchronized clocks of the measuring computers are usedas the time sources of the second highest accuracy.

In one embodiment of the present invention, when no signal of thesatellite system is present at the local GPS receiver of a firstmeasuring computer, the local clock of the first measuring computer issynchronized via NTP (Network Time Protocol) to the local clock of atleast one predetermined second measuring computer after a predeterminedtime interval—external synchronization. This has the advantage that whenthe GPS clock at a measuring computer fails for a longer period of time,which accordingly involves a failure of the internally synchronized timesource of the second highest accuracy, a third time source is generated.

According to the present invention, the time interval after which thelocal clock of the first measuring computer is externally synchronizedto the local clock of a second measuring computer is freely adjustable.

These externally synchronized local clocks of the measuring computersare used as the time sources of the third highest accuracy.Unsynchronized local clocks of the measuring computers are accordinglyreferred to as time sources of the fourth highest order.

To ensure high accuracy in the external synchronization of a local clockof a measuring computer, the external synchronization of the local clockof the measuring computer is done only with time sources of the secondhighest accuracy.

Interpretation of the accuracy of the generated time stamp is madepossible primarily in that when the local clock of a measuring computeris internally or externally synchronized, the respective synchronizationtype is stored as well as the synchronization accuracy obtained in theprocess.

According to one embodiment of the present invention, measurementpackets, in particular UDP measurement packets (User Datagram Protocol),are transmitted between the measuring computers for delay measurement.UDP is a connectionless Internet transport protocol that is based on thebasic protocol for data transmission in the Internet (IP). Preferably,the one measuring computer is used as a sender while the other measuringcomputer acts as a receiver.

The sending measuring computer records the time of departure—send timestamp—of the outgoing measurement packet. Other data associated with thesend time stamp is generated and transmitted to the receiving measuringcomputer along with the measurement packet and, possibly, further data,such as the sequence number, or the like.

Preferably, the data associated with the send time stamp relates toinformation about the used time source from which the send time stampwas read, the type of synchronization, the accuracy of thesynchronization, as well as an estimate of the accuracy of the generatedsend time stamp.

Correspondingly, the receiving measuring computer records the time ofarrival of the measurement packet—receive time stamp—as the second data,and generates other data associated with the receive time stamp.

Preferably, the data associated with the receive time stamp in turnrelates to information about the time source used for reading thereceive time stamp, the type of synchronization, the accuracy of thesynchronization, as well as an estimate of the accuracy of the generatedreceive time stamp.

Preferably, the first data and the second data are assigned to apredetermined evaluation, which may result in that these first andsecond data are not further considered when quality falls below apredetermined level.

The measurement result is determined from the still existing first dataand the second data.

BRIEF DESCRIPTION OF THE DRAWING

Further advantages, features and possible uses of the present inventionfor time synchronization in at least two measuring computers cooperatingover a telecommunications network such as Internet, intranet or similar,will become apparent from the following description in conjunction withthe exemplary embodiment shown in the drawing.

In the Drawing,

FIG. 1 is a schematic representation of a telecommunications networkincluding a plurality of measuring computers having different timesources for carrying out the method according to the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically shows a telecommunications network 10 including aplurality of switching devices 12 through 24 interconnected via trunklines 26. Telecommunications network 10 is, for example, the Internet.

Switching exchange 12 is assigned a first measuring computer 28. Toreceive signals emitted by a satellite system (GPS) including aplurality of satellites 30, first measuring computer 28 has a GPSantenna 32 and a GPS map for processing the received signals. GPSantenna 32 and the GPS map, which is not explicitly shown, together formthe local GPS receiver of first measuring computer 28 required toreceive the GPS signals. Moreover, a local clock 34 is incorporated infirst measuring computer 28.

A second measuring computer 36 connected to switching device 16 also hasa GPS antenna 38 and a local clock 40. The local GPS receiver of secondmeasuring computer 36 required to receive the GPS signals is, in turn,made up of GPS antenna 38 and a GPS map, which is integrated in secondmeasuring computer 36.

Corresponding peripheral devices, namely a GPS antenna 42 and a localclock 44, are associated with a third measuring computer 46 connected toswitching device 20. Here too, a GPS map and GPS antenna 42 form a localGPS receiver of third measuring computer 46 required to receive theemitted GPS signals.

Measuring computers 28, 36 and 46 continuously receive UTC time(Universal Coordinated Time) via the local GPS receivers introducedearlier. For the sake of simplicity, the GPS receivers of measuringcomputers 28, 36, 46 are referred to as GPS clock, as mentioned above.

Trunk lines 26 from first measuring computer 28 via switching devices12, 14 and 16 to second measuring computer 36 form a measurement path48, which is shown in the drawing as a double dot-dashed line for thepurpose of illustration.

A control computer 50 interacting with a database 52 is assigned toswitching device 24. Control computer 50 is used to control measuringcomputers 28, 36.

To carry out the measurement, a measurement program for measuring theone-way delay is installed in each of measuring computers 28 and 36.

The goal of the measurement system is to determine the packet delay of ameasurement packet from first measuring computer 28 via measurement path48 to second measuring computer 36. Thus, the measurement connection isa unidirectional measurement connection, where separate measurementpackets are sent from first measuring computer 28 to measuring computer36.

The measurement of the one-way delay is carried out according to thefollowing simplified scheme:

A measurement packet is sent from first measuring computer 28 to secondmeasuring computer 36 via measurement path 48, i.e. via trunk line 26,switching exchange 12, switching exchange 14, and switching exchange 16.In the process, the measurement packets are dispatched using the UserDatagram Protocol (UDP). UDP is a connectionless Internet transportprotocol based on IP. The measurement packets contain, inter alia, timestamps and sequence numbers.

Shortly before first measuring computer 28 sends the first bit of themeasurement packet, the so-called “send time stamp” is read out/set.This value of the send time stamp, i.e., the sending time of themeasurement packet, is transmitted to second measuring computer 36together with the measurement packet.

At second measuring computer 36, the arrival of the measurement packetis detected. In the process, a so-called “receive time stamp” isgenerated shortly after the last bit of the test packet is received atsecond measuring computer 36.

The measurement result sought, i.e., the one-way delay, correspondsroughly to the difference of the two time stamps, and is stored bycontrol computer 50 in database 52 for later visualization.

In order to minimize the probability of measurement failure caused bythe lack of a time stamp, a plurality of different time sources withgraded accuracy, which are accessible by measuring computers 28, 26 and46 for generating the time stamps, are configured as will be explainedhereinafter. However, the system always first attempts to read the timestamp from the time source of the highest accuracy.

The already described GPS clocks of measuring computers 28, 36 and 46are used as the time sources of the highest accuracy. Using the GPSclocks, measuring computers 28, 36 and 46 can generate time stamps withan error of ±0.5 μs.

The time sources of the second highest accuracy available to measuringcomputers 28, 36 and 46 are their local clocks 34, 40 and 44, which arecontinuously synchronized via NTP (Network Time Protocol) to the GPSclock or the local GPS receiver for this purpose. The synchronization oflocal clocks 34, 40 and 44 via NTP to the local GPS receivers ofmeasuring computers 28, 36 and 46 is more simply referred to also as“internal synchronization” here. In the drawing, the internalsynchronization of local clock 34 of first measuring computer 28 issymbolized by an arrow 54. At second measuring computer 36, the internalsynchronization of local clock 40 to the local GPS receiver of secondmeasuring computer 36 is symbolized by arrow 56, and at third measuringcomputer 46, the internal synchronization of local clock 44 to the localGPS receiver of third measuring computer 46 is symbolized by arrow 58.

The time sources used as the time sources of the third highest order arelocal clocks 34, 40 and 44 of measuring computers 28, 36, 46, which aresynchronized via NTP to the internally synchronized clock of the othermeasuring computer 28, 36, 46 for this purpose. In the following, thisfurther synchronization is also referred to as “externalsynchronization”, and will be further explained hereinafter.

For example, at second measuring computer 36, reception of the GPSsignals is not possible, for example, due a defective GPS antenna 38. Asa consequence, after some time, it is no longer possible to synchronizelocal clock 40 internally. In the drawing, the failure of the internalsynchronization is indicated by reference numeral 60. Then, local clock40 is externally synchronized via NTP to the internally synchronizedlocal clock 44 of third measuring computer 46, which is shown in thedrawing by broken line 62.

The unsynchronized local clocks 34, 40 and 44 of measuring computers 28,36 and 46 are referred to as time sources of the fourth highest order.

In the present example, first measuring computer 28 reads the send timestamp from the GPS clock, i.e., the clock of the highest accuracy. Thissend time stamp is written into the measurement packet. Then, the status“time stamp GPS accurate” is stored in a status field.

First measuring computer 28, i.e., the sending measuring computer, andsecond measuring computer 36, i.e., the receiving measuring computer,each have a separate region available in the status field for theirstatus entries.

If, as in the present example, the GPS clock at second measuringcomputer 36 fails, no receive time stamp can be read from the timesource of the highest accuracy. Therefore, the measurement program readslocal clock 40 of second measuring computer 36. In the process, themeasurement program detects whether local clock 40 is synchronized, thesource to which NTP synchronizes, and the accuracy of thesynchronization. Since NTP maintains the status of an internalsynchronization for several minutes, the time stamp read is almost asaccurate as the time stamp of a GPS clock. If the accuracy read is lessthan 1 millisecond, the value “NTP synchronized, accurate” is written tothe status field. If the accuracy read is less than 2 milliseconds, thenthe value “NTP synchronized, inaccurate” is written to the status field.

If the GPS clock could not be read for a longer period of time, forexample, more than about 5 minutes, then NTP automatically switches toexternal synchronization. In this mode, the accuracy of the time stampsread is clearly worse than in the case of internal synchronization.Therefore, the system only checks whether the accuracy of NTP is less 2milliseconds. Then, “NTP synchronized, inaccurate” is written to thestatus field.

If the GPS clock cannot be read, and the accuracy of NTP is worse than 2milliseconds, then the time stamp of local clock 40 of second measuringcomputer 36 is actually written into the measurement packet, but aspecial value is written to the status field, so that this measurementpacket will not be considered in the later evaluation for delaycalculation.

Accordingly, the following status field entries are generated as afunction of the time source used and the obtained accuracy:

Status field entry/ Synchronization time stamp accuracy GPS GPS accurateNTP internal synchr., accuracy < 1 ms NTP accurate NTP internal synchr.,accuracy < 2 ms NTP inaccurate NTP internal synchr., accuracy > 2 ms Notsynchronized NTP external synchr., accuracy < 2 ms NTP inaccurate NTPexternal synchr., accuracy > 2 ms Not synchronized No synchronizationNot synchronized

It is a feature of the present invention that it allows a time stamp tobe read from a different time source when the GPS clock fails, thusminimizing the probability of a measurement failure due to the lack of atime stamp.

1. A method for time synchronization of a plurality of measuringcomputers cooperating over a telecommunications network, the methodcomprising: providing a plurality of first time sources associated witha first measuring computer, each of the plurality of first time sourceshaving a different respective accuracy and configured to provide a firsttime stamp; selecting, using the first measuring computer, a third timesource of the plurality of first time sources as a function of anaccuracy of the third time source; transmitting measurement packetsbetween the first measuring computer and a second measuring computer ofthe plurality of measuring computers; generating first data associatedwith the first time stamp, the first time stamp being a send time stamp;generating second data associated with a receive time stamp; assigningthe first data and the second data to a predetermined evaluation; andstopping the evaluation of the first data and the second data when arespective quality of the first and second data falls below apredetermined level.
 2. The method as recited in claim 1 wherein thetelecommunications network includes at least one of an internet and anintranet.
 3. The method as recited in claim 1 further comprisingperforming a measurement method using the first time stamp.
 4. Themethod as recited in claim 1 wherein the third time source is moreaccurate than at least one other of the plurality of first time sources.5. The method as recited in claim 1 wherein the third time source has anext best accuracy relative to a fourth time source of the plurality offirst time sources, and further comprising attempting, using the firstmeasuring computer, to initially select the fourth time source beforethe selecting the third time source, the selecting the third time sourceincluding automatically selecting the third time source when the fourthtime source has failed.
 6. The method as recited in claim 1 wherein thethird time source includes signals of a satellite system, the third timesource being more accurate than any other of the plurality of first timesources.
 7. The method as recited in claim 6 wherein the satellitesystem includes a global positioning system.
 8. The method as recited inclaim 6 wherein the first measuring computer includes a local globalpositioning system receiver integrated therein and configured to receivethe signals of the satellite system.
 9. The method as recited in claim 1wherein each of the measuring computers includes a respective localclock continuously synchronized to a respective local GPS receiver via anetwork time protocol so as to provide a respective internallysynchronized local clock.
 10. The method as recited in claim 9 wherein afourth of the plurality of first time sources includes signals of asatellite system, and the third time source includes the internallysynchronized local clock of the first measuring computer, the third timesource having a next highest accuracy relative to the fourth timesource.
 11. The method as recited in claim 1 wherein the first measuringcomputer includes a first local global positioning system receiver andfirst local clock, and further comprising, when no signal of a globalpositioning system is present at the first local global positioningsystem receiver, synchronizing the first local clock via a network timeprotocol to a second local clock of at least one predetermined secondmeasuring computer of the plurality of measuring computers after apredetermined time interval so as to provide an externalsynchronization.
 12. The method as recited in claim 11 wherein the timeinterval is adjustable.
 13. The method as recited in claim 11 whereinthe second local clock has a second highest accuracy relative to anaccuracy of other time sources of the plurality of first time sources.14. The method as recited in claim 1 wherein the first measuringcomputer includes a first local clock and further comprisingsynchronizing the first local clock via a network time protocol to asecond local clock of at least one predetermined second measuringcomputer of the plurality of measuring computers after a predeterminedtime interval so as to externally synchronize the first local clock, theplurality of first time sources including the externally synchronizedfirst local clock, the externally synchronized first local clock havinga third highest accuracy relative to other time sources of the pluralityof first time sources.
 15. The method as recited in claim 1 furthercomprising synchronizing a first local clock of the first measuringcomputer via a network time protocol and storing a type and an accuracyof the synchronizing.
 16. The method as recited in claim 1 wherein thefirst measuring computer includes a first local clock, the first timesource including the first local clock, the first local clock beingunsynchronized, the unsynchronized first local clock having a fourthhighest accuracy relative to other time sources of the plurality offirst time sources.
 17. The method as recited in claim 1 wherein themeasurement packets include user datagram protocol packets.
 18. Themethod as recited in claim 1 wherein the first measuring computer actsas a sender and the second measuring computer acts as a receiver. 19.The method as recited in claim 1 further comprising, using the firstmeasuring computer: recording the first time stamp, the first time stampbeing a send time stamp of an outgoing measurement packet; andtransmitting the first data to the second measuring computer with anoutgoing measurement packet.
 20. The method as recited in claim 19wherein the first data relates to information about at least one of thethird time source, a type of synchronization, an accuracy of thesynchronization, and an accuracy of the send time stamp.
 21. The methodas recited in claim 19 further comprising generating, with the secondmeasuring computer, the receive time stamp, the receive time stamp beinga time stamp of an incoming measurement packet.
 22. The method asrecited in claim 21 wherein the data associated with the receive timestamp relates to information about at least one of the third timesource, a type of synchronization, an accuracy of the synchronization,and an accuracy of the receive time stamp.
 23. The method as recited inclaim 1 further comprising transmitting a sequence number to the secondmeasuring computer with an outgoing measurement packet.
 24. The methodas recited in claim 1 further comprising: determining a measurementresult from the first and second data.
 25. The method as recited inclaim 1 further comprising providing a plurality of second time sourcesassociated with a second measuring computer of the plurality ofmeasuring computers, each of the second time sources having a differentrespective accuracy and configured to provide a second time stamp.
 26. Atime synchronization device comprising: a first measuring computer; asecond measuring computer cooperating with the first measuring computerover a telecommunications network; the first measuring computerconfigured at least to generate first data associated with a second timestamp and the first data to the second measuring computer over thetelecommunication network; the second measuring computer configured atleast to receive a transmission over the telecommunication network, andto generate second data associated with a receive time stamp; each ofthe first measuring computer and the second measuring computer furtherconfigured to stop an evaluation of the first and second data when arespective quality of the first data and the second data falls below apredetermined level; and a plurality of first time sources associatedwith a first measuring computer, each of the plurality of first timesources having a different respective accuracy and configured to providea first time stamp; wherein the first computer is further configured toselect a third time source of the plurality of first time sources as afunction of an accuracy of the third time source.
 27. The timesynchronization device as recited in claim 26 further comprising aplurality of second time sources associated with the second measuringcomputer, each of the second time sources having a different respectiveaccuracy and configured to provide a second time stamp.
 28. The timesynchronization device as recited in claim 27 wherein the third timesource includes signals of a satellite system, the third time sourcebeing more accurate than any other of the plurality of first timesources.
 29. The time synchronization device as recited in claim 28wherein the satellite system includes a global positioning system. 30.The time synchronization device as recited in claim 26 wherein thetelecommunications network includes at least one of an internet and anintranet.
 31. The time synchronization device as recited in claim 26wherein the first time stamp is usable for performing a measurementmethod.
 32. The time synchronization device as recited in claim 26wherein the third time source is more accurate than at least one otherof the plurality of first time sources.
 33. The time synchronizationdevice as recited in claim 26 wherein: the third time source has a nextbest accuracy relative to a fourth time source of the plurality of firsttime sources; and the first measuring computer is configured toinitially attempt to select the fourth time source before selecting thethird time source, and then to automatically select the third timesource when the fourth time source has failed.